I’ve dealt with Benelliparts for over a decade now and never had any negative experiences like that.
Your receiver extension looks fine. They’re all discolored like that. Some of it is from heat treating on the bearing surfaces. Rubbing some oil on it will cover up most of it. The finish on the oem magazine tubes isn’t all that great to be honest.
And here it is most the way finished. A Pachmeyer decelerator will go on the end. I can fit an extra charging handle and repair parts in the new "cubby" ?